Coking emission reduction

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Emission reduction – the primary goal of technological advancement in Germany’s coking industry. Cai Chengyou, Chen Jianwei (MCC Coke & Refractory Engineering Co., Ltd., Anshan 114002). Germany is the birthplace of modern coking technology and also possesses the most advanced technologies for recovering by-products from coke ovens today. The super-large capacity coke ovens at the Schwelgen coking plant, which came online in 2003, have an effective volume per carbonization chamber of up to 93 m3, making them some of the most technologically advanced coke ovens in the world today (Figure 1). Looking back at the development history of German coking technology, it can be seen that the early 1980s marked the period of fastest growth in this field in Germany. During this period, reducing pollutant emissions and improving the environmental performance of coke ovens have become the main goals of technological advancement in German coke oven technology, as well as the most significant manifestation of such advancements. Over the past 20 years or so, the emission reduction achievements achieved by Germany through advances in coking technology are mainly reflected in the following aspects. 1 Ultra-large capacity coke ovens – the fundamental solution for emission reduction. The industrialization of ultra-large capacity coke oven technology in Germany began in the early 1980s; the projects that have been built and put into operation are listed in Table 1. The main purpose of making German coke ovens super-large is to significantly reduce pollutant emissions and improve environmental benefits. Because during the production process in coke ovens, it is impossible to completely prevent the leakage of pollutants from various sealing points, as well as the emission of pollutants during coal loading, coke pushing, and coke quenching. By making coke ovens super-large, it is possible to significantly reduce various sealing lengths as well as the number of times coal is loaded, coke is pushed out, and coke is quenched, all while maintaining production levels. This represents a fundamental solution for drastically reducing pollutant emissions. Figure 1: Schwelgen Coking Plant in Germany. Table 1: Technical parameters of ultra-large capacity coke ovens in Germany. Parameter: Mannesmann Hohengönheim Plant, Ruhrkohle Parospa Plant, Ruhrkohle Kaiserslautern Plant, Thyssen-Krupp Schwelgen Plant. Charring chamber length, mm: 18,000; 16,600; 18,800; 20,800. Charring chamber height, mm: 7,850; 7,100; 7,630; 8,430. Charring chamber width, mm: 5,500; 5,900; 6,105; 5,900. Volume per chamber, m3: 70; 62.37; 8.99; 93. Coke output per oven, t: 433; 847.85; 55. Coking time, h: 22.42; 25; 26; 25. Date of commissioning: 1984; 1985; 1992; 2003. 2. Emission reduction in the coking production process. The possible sources of various pollutants during the coking production process are shown in Figure 2. Thanks to advancements in processes, equipment, and environmental protection technologies, German coking plants have improved their control techniques for various pollutants, achieving good results in reducing emissions. 2.1 Emission reduction during coal charging Before the 1980s, the commonly used dust control technology for coal charging in German coking plants was on-board washing or combustion systems to purify the dust emitted during coal charging. Due to the low capture rate and purification efficiency, pollutant emissions during coal loading remain as high as 20 g/t of coke. Over the past 20 years, thanks to the development of the technology that combines high-pressure ammonia water injection with the introduction of coal-loading flue gas into adjacent carbonization chambers (CGT, Figure 3) and the pressure control technology for individual carbonization chambers (PROven), dust emissions during coal loading have been reduced to 1 g/t of coke, when equipment is properly maintained; accordingly, BaP emissions have also been reduced by over 95%. 2.2 Emission reduction during the coking process Before the 1980s, German coking plants basically did not have any dust removal facilities for coking, with only a few equipped with side covers over the coking area. During the coking process, pollutants primarily in the form of dust are released in an unorganized manner, with emission levels exceeding 700 g/t of coke. Over the past 20 years, German coking plants have generally been equipped with a coke pushing dust control system (PECS), which consists of coke cars equipped with smoke collectors (known as Coke Transfer Cars or CTC), belt-sealed dust collection ducts (the MinisterStein system), and surface bag filters. This system has been continuously improved, resulting in a significant reduction in pollutant emissions during coke extraction. The dust emission during the coking process is reduced to below 5 g/t of coke, meeting the requirements of Germany’s Clean Air Act (TA-Luft). At the same time, the dust concentration in the exhaust gases after treatment by a bag filter can be reduced to below 20 mg/m3. Figure 2: Sources of pollutant emissions during the coking production process. Figure 3: Technology for directing furnace gas to adjacent carbonization chambers (CGT). 2.3 Emission reduction during the quenching process. In the 1980s, German coking plants installed dry quenching equipment, but did not have a proper dust removal system. Therefore, dust pollution in areas such as the loading device, coking discharge device, and coke transportation system remains severe. The Caesars Sturgeon coking plant, which came online in 1992, uses a dry quenching system on its 7.63m coke ovens and is equipped with a relatively advanced and comprehensive dust removal system; however, the actual capacity of the dry quenching system is only 200t/h. As a result, the coking quenching process alternates between dry and wet quenching, and the overall environmental benefits of dry coking technology are not evident. Furthermore, due to energy policies, the dry quenching technology has not been promoted or developed in Germany. Over the past 20 years, Germany has focused on developing low-pollution wet quenching technology for coking, which combines the stable-quenching method (CSQ) with efficient dust collection from the quenching steam, and this technology has been successfully applied to the wet quenching system of the 8.43m coke ovens at the Schwelgen plant. The dust emission from coking steam has decreased from over 100 g/t of coke in 1980 to currently less than 25 g/t of coke on average, with the optimal operating values being below 20 g/t of coke. 2.4 Emission reduction in the carbonization chamber during coking The emission reduction in the carbonization chamber during coking depends mainly on the effectiveness of the sealing of equipment such as the furnace door, coal charging hole covers, and rising pipe covers. The harmful substances leaking from the seals of these devices are mainly polycyclic aromatic hydrocarbons (PAHs), among which benzo[a]pyrene (BaP) is the most toxic. However, the part with the highest leakage during coking is the furnace door. Over the past 20 years, the main achievement in advances in furnace door technology in Germany has been the replacement of rigid furnace doors with flexible furnace doors (Fleet) that can maintain good sealing performance under high temperatures and fluctuating temperature conditions; combined with PROven technology, this has led to a **reduction in BaP emissions. It is reported that when a rigid furnace door is used, the amount of BaP emitted from the furnace door is approximately 100 mg/t of coke. With the use of an elastic furnace door, it can be reduced to a level of just a few milligrams. With the PROven technology, PAH or BaP emissions during the coking process can be reduced by 70% (Figure 4). The application of these new technologies may reduce the amounts of BaP and benzene compounds emitted from the furnace door to optimal levels of 1 mg/t of coke and 1 g/t of coke, respectively. Figure 4: Aromatic hydrocarbon (PAH) emissions before and after the adoption of a pressure control system in the carbonization chamber. Emission reductions in the coke oven heating system: The main pollutants in the exhaust gases emitted by the coke oven heating system are SO2 and NOx. Among them, the SO2 emissions during the heating with coke oven gas mainly depend on the hydrogen sulfide content in the desulfurized coke oven gas. After the 1980s, German coking plants widely adopted the ammonia-sulfur cycle washing (A.S.) process and the vacuum potassium carbonate (Vacasulf) process to desulfurize and purify coke oven gas. The hydrogen sulfide content in the coke oven gas used for heating the coke ovens could be reduced to below 500 mg/m3 as specified by German standards, thereby lowering the average SO2 level in the exhaust gases from coke burning from around 1400 g/t of coke to below 70 g/t of coke. Regarding the concentration of nitrogen oxides (NOx) in the exhaust gases emitted by coke ovens’ combustion systems, the Clean Air Act (TA-Luft) enacted in Germany in the early 1980s stipulated that the NOx concentration in the exhaust gases released from coke oven chimneys should be below 500 mg/m3. This requirement was very high for the coke ovens in Germany that were heated with coke oven gas at that time. To meet the aforementioned standards, Germany has developed the “Combiflame” system, which combines exhaust gas recirculation with staged supply of combustion air; at a flue gas temperature of 1300°C, the NOx content in the exhaust gases can be reduced to below 350 mg/m3. As a result, the average amount of NOx emitted in the coke oven exhaust was reduced from about 1800 g/t of coke to below 600 g/t of coke. 3 Conclusion China is now the world’s largest producer of coke. Overall, China’s coking industry remains a highly polluting sector, and it is currently the country with the highest total emissions of pollutants during the coking production process in the world. Among them, the fact that the development of environmental protection technologies for coke ovens lags behind that of process production technologies is also an important reason. Germany’s focus on advancing coking technology with the primary goal of reducing pollutant emissions during the coking process since the 1980s provides us with valuable insights that can serve as a model. In today’s context of rapid development in China’s coking industry, developing efficient and environmentally friendly technologies should be an urgent task for advancing the technological level of this sector. Only through such technological progress can the highly polluting conditions associated with coking production be fundamentally changed, thereby transforming China’s coking industry into a clean, sustainable manufacturing sector.